1. Field of the Invention
The present invention relates to a digital still camera for recording a subject image as digital image data.
2. Description of the Related Art
In a digital still camera, an image sensor, such as a CCD (Charge-Coupled Device), is arranged at the rear of a photographing optical system, thus an object image is formed on the image sensor and is then recorded in a recording medium, such as a card memory. Further, an AF (Auto Focus) mechanism is provided in the camera. A focused subject image is obtained by the AF mechanism.
For the focus detecting method, a contrast detecting method, which detects a focus point in accordance with a contrast of the subject image, is usually applied. In this case, the image sensor serves as a photo-sensor for detecting the focus. However, when the subject image is not in an in-focus situation, it cannot be determined whether the subject image is focused in the background or foreground with respect to a focal plane. Therefore, to focus the subject image, focus lenses are shifted along an optical axis of the photographing optical system and the defocusing situation is determined in accordance with a contrast change caused by shifting the focus lenses. Therefore, a focus adjustment cannot be rapidly performed, especially when the subject is a moving-subject, or when a telephoto lens is used.
On the other hand, for another focus detecting method, a phase difference detecting method or a so called “phase-matching method” is known. This method is mainly applied in an SLR (Single Lens Reflex) type camera and is superior to the contrast focus detecting method. However, when constructing a SLR type of digital still camera by using an image sensor in place of a photographic film, various construction elements, such as a pentagonal roof prism, quick return mirror and sub-mirror, must be incorporated in the digital camera so that an inner construction of the camera becomes complicated and the assembly of the digital camera becomes troublesome.